Support git worktrees in RAPIDS script generation

[bdice]

Summary

• Detect checked-out RAPIDS repos with git -C <path> rev-parse --is-inside-work-tree instead of testing for a .git directory
• Allow RAPIDS build utility scripts to recognize git worktrees, where .git is a file instead of a directory
• Update the shared clone helper so local worktree origins and existing worktree targets are treated as git repositories
• Bump rapids-build-utils from 26.8.4 to 26.8.5 and utils from 26.8.1 to 26.8.2

Motivation

This is necessary to support devcontainers that use git worktrees. Worktree checkouts store .git as a file pointing at the real git metadata, so the previous -d <repo>/.git checks treated valid worktree repos as missing. In rapids-generate-scripts, that meant only aggregate/no-op scripts were generated.

Testing

• bash -n passed for the 10 modified shell scripts
• git diff --check passed
• confirmed there are no remaining .git directory predicates in the repo
• created a temporary git worktree inside the devcontainer and verified the updated clone helper accepts it as a local git repository
• patched the installed script in an RMM worktree devcontainer and verified rapids-generate-scripts --repo rmm generated build-rmm and test-rmm-python
• in the RMM worktree devcontainer, build-all -j0 passed
• in the RMM worktree devcontainer, test-all-python passed: 1780 passed, 6 skipped

[vyasr]

Seems robust enough for the devcontainers use case.

[trxcllnt]

Need to bump the feature version, otherwise looks fine to me. Are there any other places that check for -d .git that also need to be updated?